1. FIELD OF THE INVENTION
This invention relates to a control apparatus for engine-driven permanent magnet type synchronous generators, adapted to control a voltage generated by a permanent magnet type synchronous generator driven by an automobile engine, and convert the DC power of a battery into commercial AC power.
2. DESCRIPTION OF THE PRIOR ART
In an AC generator driven by an automobile engine, a generated voltage varies in proportion to the rotational frequency of the engine which varies very widely from 500 rpm to several thousand rpm. Therefore, in order to use this generator as a commercial power source, a constant voltage control unit comprising a voltage step-up means and a voltage step-down means, and an inverter circuit for converting DC power into AC power of a desired frequency are required.
An example of a conventional constant voltage control unit is shown in FIG. 3. As shown in FIG. 3, the constant voltage control unit comprises a voltage step-down circuit 2 and a voltage step-up circuit 3. The voltage step-down circuit 2 is adapted to decrease a generated output voltage of a permanent magnet type generator 1, and comprises diodes D8, D9, D10 and thyristors S1, S2, S3, the gates of the thyristors S1, S2, S3 being controlled by a voltage step-down control circuit 4, whereby a voltage step-down control operation is carried out by reducing a conducting angle of a rectifying circuit.
The voltage step-up circuit 3 is adapted to increase a generated output voltage of the permanent magnet type generator 1, and comprises a leakage inductance of an armature coil 14 of the generator 1, diodes D8, D9, D10, D11, D12, D13, and switching transistors T8, T9, a voltage step-up control operation being carried out by controlling the switching transistors T8, T9 by a voltage step-up control circuit 5.
The constant voltage control unit for the generator 1 uses the thyristors S1-S3 and switching transistors T8, T9, and these elements are large current switching elements. Since a control circuit for controlling the large current switching elements are required, the price of the control unit becomes high, and a space necessary for installing these parts increases. During an operation of the voltage step-up control circuit, the diodes D8, D9, D10, D11, D12, D13 form a series circuit to cause a loss occurring in the elements to increase, and an efficiency to decrease. The switching transistor T8 connected in series to the voltage step-up control circuit also constitutes a factor of the increase of the loss, and a switching operation of the switching transistor T9 is unstable in some cases, a special protective circuit being required for the switching transistor T8.
Japanese Patent Laid-Open No. 237998/1996 discloses a voltage control unit for a permanent magnet type three-phase AC generator, which is driven by an automobile engine without using a large current reactor, capable of obtaining a stable generated voltage of the generator at any rotational frequency of the engine.
A refrigerator-mounted truck by which materials, such as perishable foods held in a refrigerating or cold insulating container are transported is provided with an electric power converter as shown in FIG. 7, which comprises a reactor or a transformer for converting the DC power (12V or 24V) of the automobile into AC power (100V or 200V), for operating the refrigerator (refer to, for example, Japanese Patent Laid-Open No. 285756/1989).
As shown in FIG. 7, when an engine 21 is operated with the vehicle traveling, a generator 1 is driven by the engine 21 and generates power to charge a battery 16 with the step-up of a voltage of the battery 16 and the conversion of DC power into AC power controlled by a voltage step-up circuit 24 and an inverter circuit 25, whereby commercial AC power is supplied to a load, such as a compressor. When the engine 21 is stopped, the step-up of a voltage of the battery 16 and the conversion of DC power into AC power are controlled by the voltage step-up circuit 24 and inverter circuit 25, and commercial DC power is supplied to the load 26.
As shown in FIG. 8, the electric power converter referred to above, which usually increases an output from a 12V or 24V generator 1 or a battery 16 to 100V or 200V by a voltage step-up circuit only, so that the voltage step-up circuit needs to comprise a voltage step-up reactor 27. Since the compressor for the refrigerator requires several KW power, the voltage step-up reactor 27 requires a current capacity of around 100 A. Consequently, the electric power converter has to comprise a large and heavy reactor.